The present invention relates to a method and apparatus for improving the printing of design images on semiconductor wafers. More particularly, it relates to determining a tilt of projection optics relative to a wafer surface for a given technology, level and lithography tool. The tilt determined compensates for non-idealities in the optics to improve image quality.
A semiconductor chip typically comprises a plurality of design levels. Each level is added successively on top of other levels to obtain a target wafer. To add a given level to a wafer comprising existing levels, the wafer is coated with a uniform film of photoresist. In a lithography or "printing" step, a mask containing an image of the design at the level to be added is interposed between the wafer and an illumination source. Ultraviolet radiation of varying wavelengths is projected from the illumination source, through the mask, and onto the photoresist on the wafer surface.
The mask defines areas where the radiation is blocked, and areas where it is allowed to pass through and form an exposure field on the photoresist. In areas where the photoresist is exposed, the solubility properties of the photoresist are changed. Effectively, the image of the design contained in the mask is transferred to the photoresist. The photoresist may subsequently be etched away to effect a physical transfer of the design into the wafer. Ions may be implanted as desired in the design to change electrical characteristics of the wafer material.
In one approach to lithographic printing, only a portion of the mask is illuminated, and the mask and the wafer are "scanned", or moved past the illumination source to transfer the mask design onto the wafer. An exposure slit interposed between the illumination source and the mask defines the portion to be illuminated and an exposure field on the wafer surface.
One parameter which is controlled on lithography systems is the tilt of the projection optics relative to the wafer. In known lithography tools such as Micrascans, for example, a tilt that controlled is "Theta-X" (.theta..sub.X), which is the tilt of the optics wave front projected by the lithography tool relative to the wafer in a direction perpendicular to a scanning direction.
Existing lithography systems typically perform control of .theta..sub.X by imaging a specific test structure, for example, a resist stud, at various focus settings along the exposure slit direction. The tilt is adjusted until the nominal focus for the test structure is nominally identical across the exposure slit. It is assumed in these existing systems that there is a unique .theta..sub.X tilt which is applicable to all image types, and that using the .theta..sub.X obtained from imaging the test structure will adequately represent the behavior of other image types.
However, in conceiving the present invention, it has been recognized that a .theta..sub.X tilt which is adjusted for such factors as image type, wafer level and technology, and lithography tool can produce better image fidelity. The adjustment of the .theta..sub.X tilt compensates for factors which otherwise would interact with the .theta..sub.X tilt to cause an imperfect transfer of the design image in the mask to the wafer.
For example, typically a mask image is several times greater than the actual physical design size and is sent through reduction lensing in the projection optics to reduce it to the necessary size for projection onto the wafer. Non-idealities in the reduction lensing and other elements in the projection optics can cause undesired variations between the mask image and the image transferred onto the photoresist across an exposure field. For example, design image features that should be identical everywhere within the exposure field may be different in shape and size depending on their location within the exposure field. Such variations can detract from performance in the final fabricated chip. Diffraction effects, when image sizes approach the wavelength of the illumination radiation, can also produce non-uniformities in printing. Outright imaging failure can also occur in the exposure field, necessitating either reworking the wafer or scrapping it.
In view of the above, a method and apparatus for improving lithographic printing by adjusting the .theta..sub.X tilt according to parameters including wafer level and technology are disclosed hereinafter.